Single crystal X-ray analysis will be continued on the enzyme:mitochondrial malate dehydrogenase (m-MDH). Two monoclinic forms, labelled A and B, are being studied. Both crystal forms have the space group P2l but differ notably in unit cell dimensions. Polymorphism in the A form manifested by small changes in scattered X-ray intensity has made it difficult to follow heavy atom effects. No isomorphic heavy atom derivatives have yet been identified for the type A crystals and soaking experiments will be carried out to identify derivatives. In the case of type B crystals, study will focus on the location of additional binding sites. Three sites per asymmetric unit have been already found for the UO2(NO3)2 derivative and four sites per asymmetric unit uncovered for the cis Pt(NH3)2Cl2 derivative. A low resolution electron density map should be obtainable during the upcoming year. Studies on the yolk lipoprotein will focus on chemical modification of the microcrystals. The modification will be aimed at the labelling of serine phosphates which belong to phosvitin. X-ray methods will be used to monitor the changes and to locate positions for the substituents.